Using my own modem gateway? Want to replace Netgear with SMC.

Anyone know if I can use my own SMC gateway? I was given the Netgear modem and my VoIP setup does not work as SIP ALG is enabled and I can't disable it. I called tech and they said they can try to have someone swap out my modem with the old SMC one but no gurantee as they are recyling those now and only using the Netgear ones.

I want to buy an old SMC on ebay. How do I find out if the one I buy is compatible with my service? Am I allowed to use my own?